Technical Abstract:
Emergent properties and cross-scale interactions are important in driving landscape-scale dynamics during a disturbance event, such as wildfire. We used these concepts related to changing pattern-process relationships across scales to explain ecological responses following disturbance that resulted in a state change in the Chihuahuan Desert. Our objective was to provide a mechanistic understanding for a large-scale perennial grass recruitment event that was unprecedented over the 100-year history of the Jornada ARS-LTER research site in southern New Mexico. This recruitment event occurred following a sequence of wet years (2004-2008) in an area that experienced gradual shrub invasion (1915-1984) and rapid coppice dune formation (1985-2000) followed by the current stable shrubland state. Long-term observations show that this grass pulse resulted in a significant increase in primary production that could not be explained by historic patterns in rainfall amount alone. In addition, a previous wet sequence of years (1983-1988) did not result in a similar broad-scale recruitment of grasses. We used multiple, long-term datasets and a model of soil water dynamics to test three scale-dependent hypotheses to explain this larger-than-expected production of grasses in the second wet period compared to the first: (1) differences in rainfall seasonality and event size affected a sequence of plant-scale processes, (2) variation in animal abundance affected plant-to-patch scale processes, and (3) differences in soil stability affected patch-scale erosional-depositional processes and spatial connectivity among patches. Our results show that complex interactions between plant- and patch-scale processes and water availability can generate unexpected landscape-scale dynamics following disturbance. A sequence of events influenced by historic legacies and current conditions interact with vegetation-soil feedbacks at plant to patch scales to generate emergent behavior at the landscape scale. We compare these dynamics for grass responses in arid shrublands to post-fire responses in other systems in order to generalize the role of cross-scale interactions, spatial connectivity, and historic legacies in governing ecological and hydrological dynamics following disturbance.